1. Field of the Invention
The present invention is directed to a solenoid valve for use with a transmission device in automotive vehicles and, in particular, to a linear solenoid valve wherein a plunger is brought into linear movement when current is applied to a coil.
2. Description of the Prior Art
A conventional solenoid valve such as one disclosed in Japanese Patent Laid-open Publication No. Hei. 6 (1994)-123378 includes a coil wound around a bobbin, first and second yokes mounting thereon opposite sides of the bobbin, respectively, first and second bearings disposed in the respective first and second yokes, and a plunger disposed within the bobbin such that opposite ends of the plunger are supported by the respective first and second bearings, the plunger being brought into linear mode movement upon excitation of the coil.
For establishing smooth or reliable movement of the plunger, magnetic isolation between the plunger and each yoke must be formed in a uniform manner. To this end, in the conventional solenoid valve, an air gap is defined between the plunger and the yoke which is in the form of an annular ring in cross-section.
On the other hand, the plunger sometimes comes out of alignment with each of the yokes or an inclination condition may occur due to an assembly error of the bearing to the corresponding yoke.
Thus, the air gap should be of a sufficient width to prevent direct contact between the plunger and each yoke even though the plunger is brought into the inclination condition.
Such an air gap of sufficient width constitutes dead space in the solenoid valve, however, resulting in a solenoid valve larger in scale.